A spin-on glass composition is a liquid, silica-based composition that can be applied to the surface of a semiconductor wafer and spun with the wafer to provide a coating with a level top surface. With this technique, the spin-on glass composition will fill in any valleys or recessed areas in the surface of the semiconductor wafer that result from the various insulating and conductive regions. The spin-on glass coating is then dried to form a solid layer and is subsequently cured at high temperatures to form a hard silica-based (glassy) layer. This hard layer may be etched in preparation for further processing.
Spin-on glass layers have been used for various purposes in semiconductor devices. For example, they have been used as planarizing layers, gettering layers for the removal of undesirable impurities, insulating layers for the isolation of multilayer metallizations, doping vehicles for semiconductor substrates and diffusion masks for enhancing contrast in photolithography techniques. Non-uniformity is undesirable for all these applications.
There are a number of materials said to be suitable for use as spin-on glass compositions. For example, U.S. Pat. No. 3,615,943 describes a coating solution that contains the reaction products of silicon tetrachloride and acetic anhydride. Alternative methods utilize tetraethoxysilane. For example, U.S. Pat. No. 3,832,202 to Ritchie describes a liquid spin-on silica source which comprises two components, one containing tetraethoxysilane, and the other comprising the reaction product of ethyl alcohol, ethyl acetate and vinyltrichlorosilane. Furthermore, U.S. Pat. No. 4,798,629 to Wood et al. describes a spin-on glass composition that comprises a mixture of tetraethoxysilane, methyltriethoxysilane and dimethyldiethoxysilane in a 2/1/1 relationship to provide a polyorganosiloxane with an atomic weight percent carbon between 25% and 8%, preferably 11% and 9%, based on hydrolyzed organosilane.
Solutions of precondensed polyorganosiloxanes have also been described as suitable spin-on glass compositions. A coating solution which contains a prepolymer of polysilsesquioxane together with a polymer obtained from tetraethoxysilane or tetramethoxysilane is described in EPA 112,168 and Kashiwagi et al., U.S. Pat. No. 4,865,649, describes a coating solution comprising a cohydrozylate of at least two alkoxysilanes where di-, tri- and tetraalkoxysilanes are utilized.
Despite the various formulations, a number of limitations exist with respect to the production and use of most spin-on glass compositions. Problems of surface damage from subsequent processing, poor adhesion and short shelf life, among others, have limited the utility of these compositions. There is a continuing effort to obtain spin-on glass compositions which overcome these limitations.
Spin-on glass layers unavoidably have some brittleness so that cracks sometimes form during subsequent processing, especially where large thicknesses are necessary to completely fill the recessed areas and provide a level surface. Thick film spin-on glass layers are also subject to detachment from the substrate during subsequent processing. The maximum thickness of coating layers obtained from commercially available spin-glass compositions is about 5,000 .ANG. for this reason. This limitation on thickness limits the applications for the spin-on glass composition. Differential thermal expansion between the spin-on glass layer and the underlying substrate contributes significantly to the cracking of the spin-on glass layer. In order to reduce the effect of differential thermal expansion, the gap between the adjacent peaks must be relatively large. This further limits the use of the spin-on glass composition to lower density (large dimension) semiconductors.
Good adhesion between the spin-on glass composition and the underlying chip surface is essential for successful chip production. Poor adhesion can result in incomplete filling of the recessed areas or surface imperfections (non-uniformity). Good adhesion has been achieved in conventional spin-on glass compositions by keeping the carbon content of the polysiloxane below 28 atomic weight %. Such a practice limits the performance of the spin-on glas composition. For example, Morimoto et al. have reported that spin-on glass planarity increases when the carbon concentration of the polysiloxane is increased from 4% to 28%, (see "Manufacturable and Reliable Spin-On Glass Planarization Process for 1 .mu.m CMOS Double Layer Metal Technology", 5th Int'l. VLSI Multilevel Interconnect Conference, Santa Clara, Calif.; Jun. 13-15, 1988).
Instability is also a problem for most spin-on glass compositions. Spin-on glass compositions typically have a shelf life of less than six months, which makes it difficult to maintain stocks of large quantities.
A further limitation on spin-on glass compositions has been that the coating layers produced have a dielectric constant which falls in the range of 4-6. This is somewhat higher than the dielectric constant of pure oxide (O.sub.2), which is 3.9. The high dielectric constant requires that there be more space between electrically isolated devices in order to provide equivalent insulating characteristics.
Another disadvantage of spin-on glass compositions has been that it is difficult to control the plasma etch rate of the layers produced since they are sensitive to the O.sub.2 concentration. A typical etching process uses a plasma such as a mixture of CHF.sub.3 and O.sub.2. When the underlying oxide is exposed, it releases additional oxygen into the plasma, causing a significant increase in the etch rate of the spin-on glass layer. This can result in the formation of recesses and a loss of uniformity in the surface.